1. Field of the Invention
The present invention relates to a surface acoustic wave device manufacturing method to be used for surface acoustic wave filters, a surface acoustic wave device manufactured thereby, and a communications equipment.
2. Description of Related Art
Recently, surface acoustic wave filters are increasingly used in various communications equipments.
According to higher frequencies and higher functions of communications equipments, it has been increasingly demanded to increase attenuation out of the bands of the surface acoustic wave filters.
FIG. 13 shows a schematic sectional view of a face-down mounting structure of a conventional surface acoustic wave device.
In FIG. 13, the reference numeral 51 denotes a piezoelectric substrate, 52 denotes a ground pad, 53 denotes a comb-shaped IDT (Inter Digital Transducer) electrode (referred to as IDT electrode) formed on the piezoelectric substrate 51, 54 denotes a conductive pattern formed on a package (mounting substrate) 57, 55 denotes a bump for connection, and 59 denotes a conductor layer formed on the back face (opposite to the surface with the IDT electrode formed) of the piezoelectric substrate 51.
In the construction of this figure, the ground pad 52 and the IDT electrode 53 are formed of, for example, Al—Cu films, and the conductive pattern 54 and the ground pad 52 are electrically connected by the bump 55 made of, for example, Au. Furthermore, by seam-welding a lid 56 via a joint layer 58, the package 57 is sealed and the airtightness of the inside housing the surface acoustic wave element is maintained.
Main causes of deterioration in out-of-band attenuation in a surface acoustic wave device with such a conventional face-down structure are, for example, increases in electrical resistance, parasitic inductances, and electromagnetic coupling between the input and output due to floating capacitances at the ground pad 52 and the IDT electrode 53 of the surface acoustic wave element, the conductive pattern 54 of the package 57, and so on.
Particularly, electromagnetic coupling between the input and the output due to floating capacitances is described.
The surface acoustic wave element is an element using a comb-shaped IDT electrode formed on a piezoelectric substrate. Normally, the piezoelectric substance shows pyroelectricity due to extreme temperature change, so that if a process involving extreme temperature change is applied during manufacturing an element having an IDT electrode on a piezoelectric substrate, spark occurs between the electrodes of the IDT electrode and breaks the element. Therefore, in order to prevent charge accumulation on the piezoelectric substrate as much as possible, a conductor layer 59 is generally formed on the entire back face of the piezoelectric substrate.
This conductor layer 59 is effective for preventing pyroelectric break-down while manufacturing the element, however, the inventors found that a capacity coupling occurs between the conductor layer 59 and the input/output electrodes of the IDT electrode 53, which deteriorates the out-of-band attenuation.
Particularly, among various surface acoustic wave devices, a duplexer that separates a signal with a transmitting side band (for example, low-frequency side band) and a signal with a receiving side band (for example, high-frequency side band) from each other is described in detail.
This duplexer is referred to as a surface acoustic wave duplexer (hereinafter, abbreviated to SAW-DPX).
In the SAW-DPX, a transmitting side band filter (hereinafter, referred to as a transmitting side filter) and a receiving side band filter (hereinafter, referred to as a receiving side filter) are formed on the same surface of the same piezoelectric substrate to realize downsizing.
However, in actuality, if the transmitting side filter and the receiving side filter are formed on the same piezoelectric substrate, isolation performance between these filters cannot satisfy the required specifications of communications terminals.
The isolation performance means the level of a signal leakage from one filter to the other filter, and such signal leakage must be minimized.
Particularly, in a duplexer, if a transmission signal with a great power amplified on the transmitting side leaks from the transmitting side filter to the receiving side filter and leaks to the receiving side, it becomes impossible to receive a receiving signal the power level of which is originally low.
Therefore, the specifications of isolation performance required for a duplexer require minimization of signal leak, and this demand remarkably increases more than in the specifications required for interstage Dual-SAW filters.
It is considered that one of the causes of deterioration in isolation performance between the filters is elastic wave leak. Particularly, in a SAW-DPX, an elastic wave excited in the IDT electrode that forms the transmitting side filter cannot be sufficiently shut in the IDT electrode, and an elastic wave that has leaked from the IDT electrode of the transmitting side filter propagates through the surface of the piezoelectric substrate, and this is received by the IDT electrode forming the receiving side filter, resulting in leak of the signal from the transmitting side filter to the receiving side filter and deterioration in isolation performance (Akinori Miyamoto, Shin-ichi Wakana, and Akio Ito, Fujitsu Laboratories Limited, “Novel optical observation technique for shear horizontal wave in SAW resonators on 42° YX-cut lithium tantalate” 2002 IEEE ULTRASONICS SYMPOSIUM-89).
In detail, it has been considered that the elastic surface wave propagation path of the IDT electrode of the transmitting side filter and the elastic surface wave propagation path of the IDT electrode of the receiving side filter are overlapped with each other on the same straight line, so that an elastic surface wave leaks from the IDT electrode of the transmitting side filter to the IDT electrode of the receiving side filter, resulting in deterioration in isolation performance.
Therefore, there has been an attempt to improve the isolation performance by shutting-off the propagation of the leak of the elastic surface wave by forming the transmitting side filter and the receiving side filter that had been formed on the same piezoelectric substrate, on separate piezoelectric substrates so as to be separated from each other.
Although, this attempt improves the isolation performance, since the transmitting side filter and the receiving side filter which had been formed originally integrally are formed separately on separate piezoelectric substrates, when the transmitting side filter and the receiving side filter are mounted on a mounting substrate, the area occupied by a region serving as a duplexer becomes larger than in the case where the transmitting side filter and the receiving side filter are integrally formed on the same piezoelectric substrate, and this cannot meet the demand for downsizing.
Therefore, it is also possible that the IDT electrodes of the transmitting side filter and the receiving side filter are arranged so that their elastic surface wave propagation paths are not overlapped with each other, that is, for example, the elastic surface wave propagation paths of the IDT electrodes are made parallel to each other. A small-sized SAW-DPX having improved isolation performance should be obtained while realizing downsizing by forming the transmitting side filter and the receiving side filter on the same piezoelectric substrate without separately forming these on separate piezoelectric substrates.
However, the isolation performance was not improved in a detailed experiment carried out by the present inventor. This means that the cause of deterioration in isolation performance is not only the elastic surface wave leak.
The present inventor found that the back face conductor layer 59 was harmful to the isolation performance of the surface acoustic wave element although it was effective for preventing pyroelectric break-down during a process of manufacturing the element.
An object of the invention is to provide a highly reliable surface acoustic wave device that can improve the out-of-band attenuation of filters, and a communications equipment using the same.
More specifically, an object of the invention is to provide a small-sized surface acoustic wave device with excellent isolation performance in which a transmitting side filter and a receiving side filter are formed on the same piezoelectric substrate without forming these on separate piezoelectric substrates, and a communications equipment using the same.